Inserter: Enable inserting a block using the inserter

[youknowriad]

In this PR, I'm enabling the possibility to effectively add new blocks using the inserter. In the same time I'm adding the "filtering" logic. You can now filter the inserter by filling the search input.

Also on a technical side:

• We have a new INSERT_BLOCK action
• I've simplified the hovered and selected reducers to only store one value (we can only have one selected or one hovered block).

[aduth]

I was thinking it might be helpful to move this logic into the block API, like:

wp.blocks.createBlock( 'core/text', {
	content: 'Hello World'
} );

What do you think?

[youknowriad]

Good idea!! 👍

[aduth]

Total aside: How do you feel about a keywords property for a block that allows it to be discovered by more than just its title? Do we also want to search slug?

[youknowriad]

I agree this is too limited, we need to think more about how we search for blocks. Keywords seem like a good idea, we could consider adding a description to the blocks.

[aduth]

Another note is we should probably extract some of these utilities as standalone functions and unit test them. Fine for another pull request.

[aduth]

Should we clear the current filter value?

[aduth]

Maybe not here, but we'll need to consider how to insert a block at a specific index, supporting behavior like in this mockup:

https://wpcoredesign.mystagingwebsite.com/gutenberg/

[youknowriad]

I think this action can be extended. For example, in #409 I'm adding an option "after" property to this action to insert after a specific block.

[aduth]

When inserting a new text block, there's an error logged to the console from TinyMCE:

tinymce.min.js?ver=4.7.3:11 Uncaught TypeError: Cannot read property 'length' of undefined

Another oddity which I expect is a consequence of the above error is that it's not possible to unselect the newly added text block.

[youknowriad]

I addressed the feedback in e943478

we now have a new utility createBlock to create a block given its slug and attributes. The factory file could contain other helpers to create a block (from another block for example etc...)

[aduth]

Works great 👍 Nice work.

[aduth]

Cool re-use 👍

[aduth]

Another note is we should probably extract some of these utilities as standalone functions and unit test them. Fine for another pull request.